During a lithography process in the manufacture of semiconductor devices, an exposure process for a resist is performed. In this exposure process, outgas from the resist adheres to an optical system of an exposure apparatus and causes contamination. Such contamination makes it difficult to perform a suitable exposure process. Therefore, cleaning of the optical system is required to remove the contamination.
However, since it is difficult to accurately determine the degree of contamination, time to start cleaning is also difficult to determine. If cleaning is started too late, a suitable exposure process cannot be performed. Or, if cleaning is started too soon, operation performance of the exposure apparatus decreases.
Therefore, a method and an apparatus which can accurately determine the degree of the contamination are required.